With explosive increase in needs of wide-band multi media communication service such as the Internet, there has been an expectation on development of an optical fiber communication system having a large capacity and advanced functions. The number of optical communication modules used for such large-sized system has steadily been increasing with megasizing of the system, so that cost and load of mounting, as well as size of the optical communication modules, have become no more ignorable relative to the entire system. Therefore, downsizing of the optical communication modules per se, integration of functions, and cost reduction are understood as being most critical subjects.
In particular, hybrid optical integrated module, having semiconductor optical elements mounted by flip-chip bonding onto a platform having an optical waveguide circuit preliminarily formed thereon, is strongly expected as an optical integration technology closest to the practical use, typically in terms of productivity.
The flip-chip bonding may get rid of a step of wire bonding, in the process of connecting a semiconductor optical element with an external circuit. A method based on wire bonding has been suffering from a problem in that the bonded portion is likely to cause degradation or variation in dynamic characteristics, and also in that high-frequency signals are more susceptible due to inductance appears on the wire. The flip-chip bonding is, therefore, a particularly effective method of mounting semiconductor elements which operate at high frequencies.
The semiconductor optical element herein may be exemplified by a semiconductor optical element 800 as shown in FIG. 15 (see Patent Document 1). The semiconductor optical element 800 has a substrate 801, and a buffer layer 802, a light absorbing layer 803, and a cap layer 804 stacked over the substrate 801. The semiconductor optical element 800 has a mesa-shaped light receiving portion 805, and a mesa-shaped pad electrode forming portion 806. The mesa-shaped light receiving portion 805 has a p-side contact electrode 805A formed on the light receiving zone thereof, and has an n-side contact electrode 805B on the zone other than the light receiving zone of the light receiving portion 805. The pad electrode forming portion 806 has a p-side electrode 806A formed therein.
The p-side contact electrode 805A and the p-side electrode 806A are connected through an interconnect 805A1. On the n-side contact electrode 805B, an n-side electrode 805B1 is formed.
In the semiconductor optical element 800, the n-side electrode 805B1, and the interconnect 805A1 connected to the p-side electrode 806A are formed on the same mesa-shaped light receiving portion 805, wherein the interconnect 805A1 is formed in a ring shape, and the n-side electrode 805B1 has an arc portion conforming to the ring-shaped interconnect 805A1.
When the semiconductor optical element 800 is mounted by flip-chip bonding, solder coated on the n-side electrode 805B1 may flow over the interconnect 805A1 connected to the p-side electrode 806A, to thereby shortcircuit the n-side electrode 805B1 and the p-side electrode 806A.
Thus-configured semiconductor optical element 800 is, therefore, generally believed as being unsuitable for flip-chip bonding.
Alternatively, a semiconductor optical element 900 configured as shown in FIG. 16 has been proposed (see Patent Document 2, for example).
The semiconductor optical element 900 has a semiconductor substrate 904, a low concentration p-type layer 905, a high concentration p-type layer 906, a light absorbing layer 907, a low concentration n-type layer 908, an n-type cap layer 909, and a n-type contact layer 910.
The semiconductor optical element 900 has a p-side electrode forming mesa 901 having a p-side electrode 901A formed on the top thereof, a light receiving mesa 902 having the light absorbing layer 907, and also having an n-side electrode 902A formed on the top thereof, and an n-side electrode forming mesa 903 having an n-side electrode 903A formed on the top thereof. The n-side electrode 903A is connected to the n-side electrode 902A.
In the semiconductor optical element 900, the individual mesas 901 to 903 are independently formed, so as to make the p-side electrode 901A and the n-side electrode 903A less likely to cause short-circuiting when the element is mounted by flip-chip bonding.
[Patent Document 1]    Japanese Laid-Open Patent Publication No. 2001-298211
[Patent Document 2]    Japanese Laid-Open Patent Publication No. 2003-332287